Marine seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) under the seafloor. This profile does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing better images of the subsurface is an ongoing process.
For a seismic gathering process, as shown in FIG. 1, a data acquisition system 100 includes a vessel 120 towing plural streamers 140 that may extend over kilometers behind the vessel. One or more source arrays 160 may also be towed by the vessel 120 or another vessel for generating seismic waves. Conventionally, the source arrays 160 are placed in front of the streamers 140, considering the traveling direction of the vessel 120. The seismic waves generated by the source arrays propagate downward and penetrate the seafloor, eventually being reflected by a reflecting structure (not shown) back to the surface. The reflected seismic waves propagate upward and are detected by detectors on the streamers 140. This process is generally referred to as “shooting” a particular seafloor area, and the area is referred to as a “cell.” However, such a method results in data having poor azimuth distribution and shoot density.
An improvement to this conventional data acquisition method is the use of wide-azimuth (WAZ) acquisition. In a typical WAZ survey, a streamer vessel and multiple sources are used to cover a large sea area, and all sources and streamers are controlled at desired depths throughout the survey. WAZ acquisition provides better illumination of the substructure and, thus, a better final image.
A further improvement is the use of plural streamer vessels and plural source vessels as illustrated in FIG. 2. FIG. 2 illustrates a seismic data acquisition system 200 that includes a first streamer vessel 202, a first source vessel 206 offset on both the inline and cross-line directions from the first streamer vessel 202, a second source vessel 208 offset on both the inline and cross-line directions from the first source vessel 206, and a second streamer vessel 204 offset on both the inline and cross-line directions from the second source vessel 208. In other words, both the streamer vessels and the source vessels are offset on both the inline (travel) direction X and the cross-line direction Y from each other.
The sources attached to the source and streamer vessels are shot in the following sequence: vessel 202 shoots its source when reaching position 202a, vessel 206 shoots its source when reaching position 206a, vessel 208 shoots its source when reaching position 208a and vessel 204 shoots its source when reaching position 204a. Note that positions 202a, 204a, 206a and 208a are aligned along a line 210 that extends along the cross-line direction. An inline distance between two vessels may be 30 m. Thus, under this scenario (sequential shooting), the data recorded by the receivers is unblended, i.e., the data does not mix up shoots from different sources. However, this sequential shooting mode has the disadvantage of the data having poor density, as illustrated by points 220.
To improve the poor density of the system 200, another system 300 was proposed as illustrated in FIG. 3. System 300 may include the same number of streamer vessels and source vessels as system 200, but the shooting (simultaneous shooting) is different. For system 300, all the vessels 302, 304, 306 and 308 shoot simultaneously (or nearly simultaneously) at locations 302a, 304a, 306a and 308a distributed along a line 310. The line 310 is not parallel to the cross-line direction Y as in FIG. 2, but rather makes a non-zero angle with the cross-line direction Y. In this way, the shoots' density is improved, as illustrated by shooting locations 320. However, the recorded data mixes up the shoots, i.e., produces blended data.
Sequential and simultaneous shooting modes have their advantages and limitations. To summarize, the main strength of the sequential shooting mode is taking advantage of existing seismic experience, where the corresponding workflow from acquisition to processing is very well-established. In return, the main weakness of the sequential shooting mode is the low shot density, especially in the case of multi-vessel operations such as WAZ acquisitions.
The main interest in simultaneous shooting mode is the gain on shot density. The fold and signal-to-noise ratio can thus be drastically improved. However the most problematic aspect of this strategy is located at the early state of processing: a tedious de-blending step is always required for velocity model building purposes.
The use of one or the other mode will provide either a simple workflow with low shot density (no simultaneous shooting of the sources) or a complex workflow with high shot density (full simultaneous shooting sources).
Thus, there is a need to find another shooting mode way that combines the advantages of sequential shooting mode with those of simultaneous shooting mode and removes or minimizes their disadvantages. Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks, and avoid the de-blending step.